Chute channel formation is a key process in the transition from a single-thread meandering to a braided channel pattern, but the physical mechanisms driving the process remain unclear. This research combines GIS and spatial statistical analyses, field survey, Delft3D hydrodynamic and morphodynamic modelling, and Pb-210 alpha-geochronology, to investigate controls on chute initiation and stability, and the role of chute channels in the planform dynamics of large, sand-bed meandering rivers. Sand-bed reaches of four large, tropical rivers form the focus of detailed investigations; the Strickland and Ok Tedi in Papua New Guinea, the Beni in Bolivia, and the lower Paraguay on the Paraguay/Argentina border. Binary logistic regression analysis identifies bend migration style as a key control on chute channel initiation, with most chute channels forming at bends that are subject to a rapid rate of extension (elongation in a direction perpendicular to the valley axis). Bend extension rates are shown to track variation in potential specific stream power, such that reaches and sub-reaches of the rivers studied fit within a planform continuum expressed though increasing bend extension rates and chute initiation frequency, and driven by increasing stream power relative to bedload calibre. Field observations of point bar geomorphology and vegetation dynamics illustrate the importance of rapid bend extension in forming wide sloughs between scroll bars that are aligned with the direction of over-bar flow, and in breaking the continuity of vegetation encroachment on point bars. Bathymetric surveys and Delft3D simulations for the Strickland River provide insight into flow and sediment division at bifurcate meander bends. Coupled with GIS analyses, these simulations show that stable chute channels have higher gradient advantages than chute channels subject to infill, but that upstream and downstream changes in bend orientation can also influence chute stability. The process of bend extension is typically associated with an increase in the chute gradient advantage, further elucidating the role of bend migration style in chute stability. At the reach scale, rivers with higher sediment loads (Qs/Q) are characterised by higher rates of chute infill. Strickland River floodplain sedimentation rates derived through Pb-210 alpha-geochronology are substantially higher adjacent to single-thread bends than adjacent to bifurcate bends, potentially due to an observed increase in channel capacity (and reduction in floodplain inundation frequency) associated with bend bifurcation. Further research is needed to determine whether this observation is significant in light of high uncertainty in the spatial variability of sedimentation rate estimates, but the data presented highlight a need for carefully considered stratified sampling approaches in floodplain coring campaigns, and illustrate the complexity of possible sediment dispersal mechanisms, and associated ecological responses. GIS analysis of the response of the Ok Tedi in Papua New Guinea to direct addition of mine tailings elucidates interplay between channel steepening due to the propagation of a tailings sediment slug, and mid-channel bar formation induced by the increased sediment load, with associated topographic forcing of bend and chute development. A temporal pattern of increased chute initiation frequency on the Ok Tedi mirrors the inter- and intra-reach spatial pattern of chute initiation frequency on the Paraguay, Strickland and Beni Rivers, where increased stream power is associated with increased bend extension and chute initiation rates. The process of chute formation is shown to be rate-dependent, and the threshold value of bend extension for chute initiation is shown to scale with reach-scale stream power, reminiscent of slope-ratio thresholds in river avulsion. However, Delft3D simulations suggest that chute formation can exert negative feedback on shear stress and bank erosion in the adjacent mainstem bifurcate, such that the process of chute formation is also rate-limiting. Chute formation is activated iteratively in space and time in response to changes in river energy, selectively targeting sites of greatest change, and thereby mediating the river response.

No description available.

"Micro aerial vehicles (MAV) are small remotely piloted or autonomous aircraft. Wingspans of MAVs can be as small as six inches to allow MAV’s to avoid detection during reconnaissance missions. Improving the aerodynamic efficiency of MAV’s by increasing the lift to drag ratio could lead to increased MAV range and endurance or future decreases in aircraft size. In this project, biologically inspired flight is used as a framework to improve MAV performance since MAV’s operate in a similar flight regime to birds. A novel wind tunnel apparatus was constructed that allows the planform shape of a MAV wing to be easily altered. The scale-model wing mimics a bird wing by using variable feather lengths to vary the wing planform shape. Genetic algorithms that use natural selection as an optimization process were applied to establish successive populations of candidate wing shapes. These wing shapes were tested in the wind tunnel where wings with higher fitness values were allowed to ‘breed’ and create a next generation of wings. After numerous generations were tested an acceptably strong solution was found that yielded a lift to drag ratio of 3.28. This planform was a non conventional planform that further emphasized the ability of a genetic algorithm to find a novel solution to a complex problem. Performance of the best planform was compared to previously published data for conventional MAV planform shapes. Results of this comparison show that while the highest lift to drag ratio found from the genetic algorithm is lower than published data, inabilities of the test wing to accurately represent a flat plate Zimmerman planform and limitations of the test setup can account for these discrepancies."

The goal of this project is to use inspiration acquired from bird flight to optimize the wing planform of micro-air vehicle wings. Micro-air vehicles are used by the military for surveillance and for search and rescue missions by civilian first-responders. These vehicles fly in the same low Reynolds number regime as birds, and have low aspect ratios similar to the pheasants and grouse of the order Galliformes. Conventional analysis is difficult for low Reynolds numbers, prompting use of biologically inspired methods of optimization. Genetic algorithms, which mimic the process of evolution in nature, were used to define wing shapes that were tested in wind tunnel experiments. In these experiments, lift-drag ratios at various angles of attack were measured on scale model micro-air vehicle wings (with variable length feathers) similar in shape to a bird wing. The planform shape of the scale model wing evolved in the wind tunnel flow over successive generations to ultimately produce superior wings with higher lift-drag ratios. The low angle of attack wings were easily optimized into a wing shape different from and potentially more efficient than the oft-used Zimmerman planform. The process was repeated for a higher angle of attack, near stall conditions, which yielded a different wing planform shape. Chord distributions of the optimized low angle of attack wings were found to closely match the same distributions of birds from the order Galliformes. Results from flow visualization studies meant to illuminate possible physics responsible for the higher lift-drag ratios were also investigated.

It is well established that the temperature gradients in the interiors of internally-heated mantle convection models are subadiabatic (e.g. Parmentier et al., 1994; Bunge et al., 1997, 2001). The subadiabatic gradients have been explained to arise due to a balance between vertical advection and internal heating, however, a detailed analysis of the energy balance in the subadiabatic regions has not been undertaken. In this research, I examine in detail the energy balance in a suite of two-dimensional convection calculations with mixed internal and basal heating, depth-dependent viscosity and continents. I find that there are three causes of subadiabatic gradients. One is the above-mentioned balance, which becomes significant when the ratio of internal heating to surface heat flux is large. The second mechanism involves the growth of the overshoot (maximum and minimum Temperatures along a geotherm) of the geotherm near the lower boundary where the dominant balance is between vertical and horizontal advection. The latter mechanism is significant even in relatively weakly internally heated calculations. For time-dependent calculations, I find that local secular cooling can be a dominant term in the energy equation and can lead to subadiabaticity. However, it does not show its signature on the shape of the time-averaged geotherm. I also compare the basal heat flux with parameterized calculations based on the temperature drop at the core-mantle boundary, calculated both with and without taking the subadiabatic gradient into account and I find a significantly improved fit with its inclusion.<p> I also explore a wide range of parameter space to investigate the dynamical interaction between effects due to surface boundary conditions representing continental and oceanic lithosphere and the endothermic phase boundary at 660 km-depth in two-dimensional Cartesian coordinate convection calculations. I find that phase boundary induced mantle layering is strongly affected by the wavelength of convective flows and mixed surface boundary conditions strongly increase the horizontal wavelength of convection. My study shows that for mixed cases the effects of the surface boundary conditions dominate the effects of the phase boundary. I show that the calculations with complete continental coverage have the most significantly decoupled lower and upper mantle flows and substantial thermal and mechanical layering. Unlike the free-slip case where the surface heat flux decreases substantially with increasing magnitude of the Clapeyron slope, surface heat flux is shown to be almost independent of the Clapeyron slope for mixed boundary condition cases. Although very different when not layered, models with free and mixed surfaces have very similar planforms with very large aspect ratio flows when run with large magnitudes of the Clapeyron slope. I also calculate the critical boundary layer Rayleigh number as a measure of the thermal resistance of the surface boundary layer. My results show that the thermal resistance in the oceanic and the continental regions of the mixed cases are similar to fully free and no-slip cases, respectively. I find that, even for purely basally heated models, the mantle becomes significantly subadiabatic in the presence of partial continental coverage. This is due to the significant horizontal advection of heat that occurs with very large aspect ratio convection cells.

It is well established that the temperature gradients in the interiors of internally-heated mantle convection models are subadiabatic (e.g. Parmentier et al., 1994; Bunge et al., 1997, 2001). The subadiabatic gradients have been explained to arise due to a balance between vertical advection and internal heating, however, a detailed analysis of the energy balance in the subadiabatic regions has not been undertaken. In this research, I examine in detail the energy balance in a suite of two-dimensional convection calculations with mixed internal and basal heating, depth-dependent viscosity and continents. I find that there are three causes of subadiabatic gradients. One is the above-mentioned balance, which becomes significant when the ratio of internal heating to surface heat flux is large. The second mechanism involves the growth of the overshoot (maximum and minimum Temperatures along a geotherm) of the geotherm near the lower boundary where the dominant balance is between vertical and horizontal advection. The latter mechanism is significant even in relatively weakly internally heated calculations. For time-dependent calculations, I find that local secular cooling can be a dominant term in the energy equation and can lead to subadiabaticity. However, it does not show its signature on the shape of the time-averaged geotherm. I also compare the basal heat flux with parameterized calculations based on the temperature drop at the core-mantle boundary, calculated both with and without taking the subadiabatic gradient into account and I find a significantly improved fit with its inclusion.<p> I also explore a wide range of parameter space to investigate the dynamical interaction between effects due to surface boundary conditions representing continental and oceanic lithosphere and the endothermic phase boundary at 660 km-depth in two-dimensional Cartesian coordinate convection calculations. I find that phase boundary induced mantle layering is strongly affected by the wavelength of convective flows and mixed surface boundary conditions strongly increase the horizontal wavelength of convection. My study shows that for mixed cases the effects of the surface boundary conditions dominate the effects of the phase boundary. I show that the calculations with complete continental coverage have the most significantly decoupled lower and upper mantle flows and substantial thermal and mechanical layering. Unlike the free-slip case where the surface heat flux decreases substantially with increasing magnitude of the Clapeyron slope, surface heat flux is shown to be almost independent of the Clapeyron slope for mixed boundary condition cases. Although very different when not layered, models with free and mixed surfaces have very similar planforms with very large aspect ratio flows when run with large magnitudes of the Clapeyron slope. I also calculate the critical boundary layer Rayleigh number as a measure of the thermal resistance of the surface boundary layer. My results show that the thermal resistance in the oceanic and the continental regions of the mixed cases are similar to fully free and no-slip cases, respectively. I find that, even for purely basally heated models, the mantle becomes significantly subadiabatic in the presence of partial continental coverage. This is due to the significant horizontal advection of heat that occurs with very large aspect ratio convection cells.

The main aim of this research is directed towards the study of thin-walled rectangular planform silos with a view to maximising their structural efficiency. In thin plates of the type making up the wall, membrane action may increase the load carrying capability and current design guides make no account of this. Designing rectangular silos with this in mind can lead to significant structural savings. The core of the research involves using the finite element method to study the patterns of pressure exerted by the weight of a granular bulk solid on the walls of the silo structure. The stored granular solid must use an elastic-plastic material law in order to account for large deformations that can occur in a thin-walled structure. The need for this type of constitutive law led to the investigation of bulk solid properties and shows that parameters that have previously been used to categorise bulk solids may not be sufficient to describe all aspects of their behaviour. The finite element model created uses material constitutive laws that can be found in a number of packages. The required granular material parameters can be determined from a number of simple tests. This approach aims to enable engineers to routinely use similar models when designing silos. The results obtained from the finite element model exhibited some anomalies that had been observed in previous work. These were mainly apparent in the form of localised pressure peaks near the base of the model. These effects were investigated and possible mechanisms that lead to them were proposed. The results from the finite element model were compared to previous experimental work and existing theories. The model was then used to conduct parametric surveys on square and rectangular planform silos and the distribution of pressure across the wall compared to previous predictive models. Finally, a scale thin-walled metal silo was constructed and pressure measurements on filling with pea gravel made. These are compared to predictions made by the finite element model.

This study demonstrates the applicability of using a modified application strategy of panel method to analyze low aspect ratio wings at preliminary design phases. Conventional panel methods fail to capture the leading edge vortex (LEV) that is shed by wings with low aspect ratios, typically below 2 depending on planform. This aerodynamic phenomenon contributes to a significant amount of the lift of these wings and the result is a drastic underestimation of the lift characteristics when analyzed by conventional panel method. To capture the effect of the leading edge vortex, a panel method code was used with an extended definition of the Kutta condition along portions of the leading edge inducing a vortex to shed from the leading edge and flow aft just inside the leading edge. To validate that this method, it was applied to 2 elliptical planforms with constant thickness where experimental force balance data was available. Additionally, the same 2 wings were analyzed using a finite volume solver to compare pressure distributions and to demonstrate the difference in magnitude of solution times. For comparison purposes, the resulting forces and moments from both computational methods and experimental testing were plotted over a range of angles of attack. Overall, the results demonstrate that a modified panel method could be used during the preliminary design phases for low aspect ratio wings. The panel method can reasonably model the lift and induced drag characteristics of low aspect ratio wings. This method loses applicability beyond the stall point where the leading edge vortex breaks down and oversimplifies pitching moment relation to angle of attack. Additionally, when compared to finite volume solutions of the same scenario, the panel method provided a result 20 to 30 times faster than the finite volume solutions. With this in mind, the modified panel method application strategy lends itself to preliminary design phases of low aspect ratio wings where the level of detail does not warrant finite volume analysis and solution speed has higher priority.

No description available.

No description available.